Atypical lipomatous tumour (ALT)

by Bibianna Purgina, MD FRCPC
July 23, 2024

Background:

An atypical lipomatous tumour (ALT) is a type of cancer made up of fat. ALTs can start anywhere in the body but the most common location for this tumour is the abdomen. Another name for an ALT is well differentiated liposarcoma.

Is an atypical lipomatous tumour a type of cancer?

Yes, ALT is a type of cancer. However, compared to other types of cancer, ALT is a very slow-growing tumour that very rarely spreads to other parts of the body.

What is the difference between an atypical lipomatous tumour and a well differentiated liposarcoma?

ALT and well differentiated liposarcoma are two names given to the same tumour. Doctors use the term ALT when describing tumours that start in a superficial part of the body, such as the arm or leg, and where the tumour can be fully removed by surgery. The term well differentiated liposarcoma is used for tumours starting in deeper locations such as the back of the abdomen and for those that cannot be fully removed by surgery alone.

How is this diagnosis made?

The diagnosis of an ALT is usually after a small sample of the tumour is removed in a procedure called a biopsy. The biopsy tissue is then sent to a pathologist, who examines it under a microscope. The diagnosis can also be made after the entire tumour is removed as an excision or resection specimen.

Microscopic features of this tumour

When examined under the microscope, an ALT comprises large cells called adipocytes or fat cells. Often, the tumour can look very much like normal fat. However, unlike normal fat, the tumour contains abnormal-looking cells adipocytes, known as lipoblasts.

atypical lipomatous tumour
Atypical lipomatous tumour. The tumour comprises abnormal or “atypical” looking fat cells.

FNCLCC grade

Pathologists divide ATLs into three grades based on a system created by the French Federation of Cancer Centers Sarcoma Group (FNCLCC). This system uses three microscopic features to determine the tumour grade: differentiation, mitotic count, and necrosis. These features are explained in more detail below. The grade can only be determined after a tumour sample has been examined under the microscope.

Points (from 0 to 3) are assigned for each of the microscopic features (0 to 3), and the total number of points determines the final grade of the tumour. According to this system, atypical lipomatous tumours may be either low or high-grade tumours. The tumour grade is important because high-grade tumours (grades 2 and 3) are more aggressive and are associated with a worse prognosis.

Points associated with each grade:

  • Grade 1 – 2 or 3 points.
  • Grade 2 – 4 or 5 points.
  • Grade 3 – 6 to 8 points.

Microscopic features used to determine the grade:

  1. Tumour differentiation – Tumour differentiation describes how closely the tumour cells look like normal fat cells. Tumours that look very similar to normal fat cells are given 1 point while those that look very different from normal fat cells are given 2 or 3 points. All ALTs receive 1 point for tumour differentiation.
  2. Mitotic count – A cell that is in the process of dividing to create two new cells is called a mitotic figure. Tumours that are growing fast tend to have more mitotic figures than tumours that are growing slowly. Your pathologist will determine the mitotic count by counting the number of mitotic figures in ten areas of the tumour while looking through the microscope. Tumours with no mitotic figures or very few mitotic figures are given 1 point, while those with 10 to 20 mitotic figures are given 2 points, and those with more than 20 mitotic figures are given 3 points.
  3. Necrosis – Necrosis is a type of cell death. Tumours that are growing fast tend to have more necrosis than tumours that are growing slowly. If your pathologist sees no necrosis, the tumour will be given 0 points. The tumour will be given 1 point if necrosis is seen but makes up less than 50% of the tumour or 2 points if necrosis makes more than 50% of the tumour.

MDM2

MDM2 is a gene that promotes cell division (creating new cells). Normal cells and those in non-cancerous tumours have two copies of the MDM2 gene. In contrast, ALTs have more than two copies of the MDM2 gene which helps the tumour cells divide faster than normal cells. A test called fluorescence in situ hybridization (FISH) is commonly used to count the number of MDM2 genes in a cell. An increased number of genes (more than two) is called amplification and supports the diagnosis of ALT.

Dedifferentiation in an atypical lipomatous tumour

Some ALTs will change over time so that some of the cells no longer look like normal fat. This process is called dedifferentiation. Finding dedifferentiation in an ALT changes the diagnosis to dedifferentiated liposarcoma. This change is important because dedifferentiated liposarcoma is a more aggressive cancer that is more likely to grow back after surgery and spread to other parts of the body.

Tumour size

Tumour size is important because tumours less than 5 cm are less likely to spread to other body parts and are associated with a better prognosis. Tumour size is also used to determine the pathologic tumour stage (pT).

Margins

In pathology, a margin is the edge of tissue removed during tumour surgery. The margin status in a pathology report is important as it indicates whether the entire tumour was removed or if some was left behind. This information helps determine the need for further treatment.

Pathologists typically assess margins following a surgical procedure, like an excision or resection, that removes the entire tumour. Margins aren’t usually evaluated after a biopsy, which removes only part of the tumour. The number of margins reported and their size—how much normal tissue is between the tumour and the cut edge—vary based on the tissue type and tumour location.

Pathologists examine margins to check if tumour cells are present at the tissue’s cut edge. A positive margin, where tumour cells are found, suggests that some cancer may remain in the body. In contrast, a negative margin, with no tumour cells at the edge, suggests the tumour was fully removed. Some reports also measure the distance between the nearest tumour cells and the margin, even if all margins are negative.

 

Margin

Pathologic stage (pTNM)

​The pathologic stage for ALT is based on the TNM staging system, an internationally recognized system created by the American Joint Committee on Cancer. This system uses information about the primary tumour (T), lymph nodes (N), and distant metastatic disease (M)  to determine the complete pathologic stage (pTNM). Your pathologist will examine the tissue submitted and give each part a number. In general, a higher number means more advanced disease and a worse prognosis.

Tumour stage (pT) for atypical lipomatous tumour

The tumour stage for ALT varies based on the body part involved. For example, a 5-centimetre tumour that starts in the neck will be given a different tumour stage than a tumour that starts deep in the back of the abdomen (the retroperitoneum). However, in most body sites, the tumour stage includes the tumour size and whether the tumour has grown into surrounding body parts.

Head and neck
  • T1 – The tumour is no greater than 2 centimetres in size.
  • T2 – The tumour is between 2 and 4 centimetres in size.
  • T3 – The tumour is greater than 4 centimetres in size.
  • T4 – The tumour has grown into surrounding tissues such as the bones of the face or skull, the eye, the larger blood vessels in the neck, or the brain.
Chest, back, or stomach and the arms or legs (trunk and extremities)
  • T1 – The tumour is no greater than 5 centimetres in size.
  • T2 – The tumour is between 5 and 10 centimetres in size.
  • T3 – The tumour is between 10 and 15 centimetres in size.
  • T4 – The tumour is greater than 15 centimetres in size.
Abdomen and organs inside the chest (thoracic visceral organs)
  • T1 – The tumour is only seen in one organ.
  • T2 – The tumour has grown into the connective tissue surrounding the organ from which it started.
  • T3 – The tumour has grown into at least one other organ.
  • T4 – Multiple tumours are found.
Retroperitoneum (the space at the very back of the abdominal cavity)
  • T1 – The tumour is no greater than 5 centimetres in size.
  • T2 – The tumour is between 5 and 10 centimetres in size.
  • T3 – The tumour is between 10 and 15 centimetres in size.
  • T4 – The tumour is greater than 15 centimetres in size.
Tissue around the eye (orbit)
  • T1 – The tumour is no greater than 2 centimetres in size.
  • T2 – The tumour is greater than 2 centimetres in size but has not grown into the bones surrounding the eye.
  • T3 – The tumour has grown into the bones surrounding the eye or other bones of the skull.
  • T4 – The tumour has grown into the eye (the globe) or the surrounding tissues such as the eyelids, sinuses, or brain.

Nodal stage (pN) for atypical lipomatous tumour

ALT is given a nodal stage of 0 or 1 based on the presence of cancer cells in a lymph node. If no cancer cells are seen in any of the lymph nodes examined, the nodal stage is N0. If cancer cells are seen in any of the lymph nodes examined, the nodal stage becomes N1.

About this article

Doctors wrote this article to help you read and understand your pathology report. If you have additional questions, contact us.

Other helpful resources

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